Although it is widely assumed that bidirectional influence accounts for the structure of early interactions, this hypothesis has never been adequately tested against alternate hypotheses. In order to evaluate this and alternate hypotheses, interactions between 54 mother-infant pairs--18 each at 3, 6, and 9 months--have been coded with a set of behavioral descriptors and a 1/4 sec time base. The interaction data will be analyzed with two separate sets of procedures that correspond to the principal ways in which behavior has been operationalized in studies of social interaction. In the first set, transition frequencies among mother-infant joint states will be analyzed with Thomas & Malone's (1979) Model 1. In the second, mother and infant scaled scores will each be scaled along an attentional/affective dimension, and these will be analyzed with Box-Jenkins bivariate time series techniques. Both sets of analyses will provide estimates of: infant sensitivity to own and other's prior response; and comparable estimates for the mother. The hypothesis of bidirectional influence will be supported at those ages for which infant and mother sensitivity to other's prior response is significant. Developmental changes in bidirectional influence will be analyzed with nonparametric techniques, for the Thomas & Malone parameter estimates, and ANOVA for the time series estimates. The proposed study will provide a comprehensive evaluation of the bidirectional and alternate hypotheses.